The present invention is related to an improved process for recovering metals, particularly precious metals, from clay-containing ores. More specifically, the present invention is related to the use of specific polymers and wetting agents for improved metal recovery from clay.
Heap leaching is an industrial mining process for the extraction of metals such as gold, silver, copper, uranium, and other compounds from ore. The process includes the use of chemical reactions to form complexes with specific minerals and then releases these minerals after their division from other earth materials. Similar to in situ mining, heap leach mining differs in that it places ore on a liner with chemicals added via drip systems to the ore.
The mined ore is usually crushed into small chunks and heaped on an impermeable plastic and/or clay lined leach pad where it can be irrigated with a leach solution to dissolve the valuable metals. While sprinklers are occasionally used for irrigation, drip irrigation is preferred to minimize evaporation, provide more uniform distribution of the leach solution, and to avoid damaging the exposed mineral. The solution percolates through the heap and leaches both the target as well as other minerals. This process, called the “leach cycle,” generally takes one or two months for simple oxide ores, such as most gold ores, to two years for other ores such as nickel laterite ores. The leach solution containing the dissolved minerals is typically collected, treated in a process plant to recover the target mineral and in some cases precipitate other minerals, and then recycled to the heap after reagent levels are adjusted. Ultimate recovery of the target mineral can range from 30% of contained ores, such as run-of-mine dump leaching sulfide copper ores, to over 90% for the easiest to leach ores such as some of the oxide gold ores.
The crushed ore is irrigated with a dilute alkaline cyanide solution. The solution containing the dissolved metals, typically referred in the art as pregnant solution, continues percolating through the crushed ore until it reaches the liner at the bottom of the heap where it drains into a storage pond which is often referred to in the art as a pregnant solution pond. After separating the metals from the pregnant solution, the dilute cyanide solution, typically referred to in the art as a “barren solution”, is normally re-used in the heap-leach-process or occasionally sent to an industrial water treatment facility where the residual cyanide is treated and residual metals are removed. In very high rainfall areas, such as the tropics, in some cases there is surplus water which may be discharged to the environment, after treatment. This practice can cause water pollution if improperly performed.
The process generates a large volume of waste material and is rather burdensome on a large scale. By way of example, the production of the equivalent of one gold ring through the heap method can generate 20 tons of waste material.
During the extraction phase, gold ions are solubilized by forming complex ions with cyanide:Au1+(s)+2CN−(aq)→Au(CN)2−(aq)According to J. B. Hiskey, Arizona Bureau of Geology and Mineral Technology Fieldnotes, Vol. 15, No. 4, Winter 1985, gold is dissolved in an aerated cyanide solution according to the following two-step reaction sequence:2Au+4CN−+O2+2H2O→2Au(CN)2−+2OH−+H2O2  (1)2Au+4CN−+H2O2→2Au(CN)2−+2OH−  (2).
Recuperation of the gold is readily achieved with a redox-reaction or with electrowinning, also called electroextraction, wherein metals are electrodeposited from their ores that have been put in solution. Electrorefining uses a similar process to remove impurities from a metal. Both processes use electroplating on a large scale and are important techniques for the economical and straightforward purification of non-ferrous metals.
In electrowinning, a current is passed from an inert anode through a liquid leach solution containing the metal. The metal is extracted as it is deposited in an electroplating process onto the cathode. In electrorefining, the anodes consist of unrefined impure metal. As the current passes through the acidic electrolyte the anodes are corroded into the solution so that the electroplating process deposits refined pure metal onto the cathodes.
Clay in ore can interfere with the leaching process. Certain clays can hydrate and swell when exposed to the aqueous leaching solution. The swollen clay particles can slow or block the flow of leaching solution through the heap and thus reduce leaching productivity. Therefore, there is a need in the mining industry for materials or processes which minimize the deleterious effects of clay swelling on heap leaching productivity.
In spite of the ongoing effort those of skill in the art still do not have a suitable option for mitigating the productivity losses associated with clay in heap leaching.